1. Field of the Invention
The present invention relates to a tool shaft for a tool of the percusive and rotative type, such as hammer drills and other percussive drills, and having a substantially cylindrical end portion adapted to be releasably received within a cylindrical bore of a tool holder having a least one driving member extending radially into the bore.
2. Description of the Prior Art
A tool shaft of this type is known for example from U.S. Pat. No. 4,047,722. The tool shaft disclosed in this patent specification is adapted to be mounted in a tool holder, which has fixed driving members extending into the cylindrical bore of the holder and forming together with the recesses of the tool shaft a kind of bayonet joint. Each of the fixed driving members has a surface part corresponding substantially to part of a cylindrical surface having an axis extending at right angles to the longitudinal axis of the bore of the holder and of a tool shaft mounted in the holder.
The said known tool shaft has two pairs of first and second recesses arranged symmetrically about the longitudinal axis of the tool shaft. Each of the first recesses is formed by a plane milled surface part and functions as a guide surface for guiding a corresponding driving member of the holder into the second recess when the shaft is inserted into the holder. Each of these second recesses has a cross-sectional profile defining a obtuse angle and is adapted to drivingly engage with a corresponding driving member of the holder. Each of the peripherally extending third recesses or transition recesses interconnecting the guiding and driving recesses of the respective pair of recesses is defined by a surface part having a cross-sectional profile in the form of an arc of a circle having a radius corresponding substantially to the radius of the cylinder-shaped driving members of the holder. When this known tool shaft is inserted into a corresponding tool holder, the tool shaft is at first displaced axially in relation to the tool holder, while the driving elements are moved along the guiding recesses or first recesses, and thereafter the tool shaft is rotated in relation to the tool holder, whereby the driving members of the holder are passed into the second recesses or driving recesses having closed opposite ends via the third recesses or transition recesses.
When the tool shaft is provided with two pairs of recesses like the known embodiment described above the guiding and driving recesses in each pair may be peripherally off-set only 90.degree. at the most. Furthermore, in order to obtain acceptable conditions for transmission of power between the driving elements of the tool holder and the tool shaft the recesses must have a certain radial depth determined by the shape and size of the parts of the driving elements extending into the bore of the tool holder. Consequently, the plane milled surface part defining the first recess or guiding recess, and the surface part defining the associated second recess or driving recess will meet or intersect along a line extending parallel to the tool axis and being spaced from that axis at a radial distance which is substantially smaller than the radius of the cylindrical outer surface of the tool shaft.
It has been found that when such a tool shaft is used in connection with a tool holder having spherical driving elements or cylindrical driving elements having their axes extending parallel to the axis of the tool holder (such as the tool holder disclosed in Austrian patent specification No. 285405) there is a certain risk that the tool shaft is inadvertently released from the holder during operation of the tool, especially when the tool is of the type exposed to percussions during operation, such as hammer drills and other percussive drills. The reason is that the axially extending shaft edge portion separating the guiding recess from the corresponding driving recess is spaced radially inwardly from the cylindrical outer surface of the tool shaft as explained above. Therefore, especially when the tool shaft has been exposed to wear, the separating shaft edge cannot always prevent the spherical or cylinder shaped driving elements from being transferred directly from the driving recess to the corresponding guiding recess without passing the transition recess, in case the tool bit is exposed to an incidental rotative force directed in the same direction as the rotative driving force transmitted through the tool holder.